ALEXANDRIA, Va. (WUSA9) -- Officials have determined that the six-alarm warehouse fire that injured a firefighter was intentional.

Fire marshals from the Alexandria Fire Department say a person of interest has been identified.

On Monday, fire crews responded to a call for a three-alarm fire at a warehouse, located on 801 Pickett Street at approximately 11:49 a.m. When firefighters entered the building a section of the roof of the building collapsed.

It was upgraded to a 6-alarm fire shortly afterward. It took hours for firefighters to put out the fire. Officials estimate the fire caused about $6 million dollars in damages.

One firefighter was injured with a minor medical condition in this incident, officials said.

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) National Response Team (NRT) also helped during the investigation.